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He pointed to Michigan, Pennsylvania and New England states as winnable

Washington (CNN)A top Donald Trump adviser on Sunday stuck by the presumptive Republican nominee's claim that he never posed as his own spokesman.

Trump convention manager Paul Manafort told CNN's Jake Tapper on "State of the Union" he believes Trump's claim that 25-year-old recordings of a publicist named "John Miller" are not -- as The Washington Post reported -- actually Trump himself.

"I couldn't tell who it is. If Donald Trump says it's not him, I believe it's not him," Manafort said.

Pressed on Trump's admissions in the past that he has used the names John Miller and John Barron, and that recordings of "John Miller" from an interview with People magazine sound like Trump, Manafort dismissed the question, noting that he's worked for Trump for six weeks and already uses language similar to Trump's.

"I just know that he said it's not him," Manafort said. "I believe him. I don't even know the relevance of this, frankly."

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Manafort also dismissed the People magazine recording because it was 25 years old, implying it therefore was irrelevant. When Tapper pointed out that Trump has hit his likely Democratic opponent, Hillary Clinton, over how she allegedly treated women who had affairs with her husband, Bill Clinton, during the 1990s, Manafort said there's a key difference.

"That was not People magazine. Those were issues that related to a core component of Hillary Clinton's strategy," he said, pointing out that Clinton is campaigning on "breaking the glass ceiling" by electing the United States' first female president.

"She was an enabler and made the victim of those dalliances into a real bad situation," Manafort said.

"Do you know how many people I have imitating my voice now? It's like everybody," he told Dowd in an interview published over the weekend.

Manafort also stuck by Trump's refusal to release his tax returns, citing Trump's explanation that his returns since 2009 are still the subject of Internal Revenue Service audits.

"This is an issue the media is interested in. It's not an issue the rest of America is interested in, frankly," Manafort said.

Asked whether there's something those returns would reveal that Trump doesn't want America to see, Manafort said: "He said there's nothing in there. I have no basis to believe otherwise."

Trump also said any claims that his tax returns contain politically damaging information are unfounded in his interview with Dowd.

"They would show, do I use Cayman Islands stuff? And the answer is no, I can tell you right now," he said. "Am I ensconced in some of the crazy countries where you keep money and avoid taxes? The answer is no, I don't do that."

In the interview, Manafort also touted Trump's ability to change the general election map.

He pointed to Michigan and Pennsylvania -- two Great Lakes states that Democrats have won in recent elections -- as Trump targets, as well as New England states like New Hampshire, Maine and Connecticut.

And Manafort dismissed Trump's high negative ratings with women, noting that Clinton is unpopular among male voters and that Trump "just came out of a very spirited (primary) contest."

"It's an issue. It'll be dealt with," he said. "We're coming to a healing process. This is when we're unifying our party."